Composite FM stereo signals as detected by an FM receiver comprise a base band (0-15 kHz) containing the sum of the left and right channel signals (L+R), a 19 kHz pilot signal and a subchannel centered at 38 kHz containing the difference of the left and right channel signals (L-R). To decode the composite signal the decoder of an FM stereo receiver must lock onto the pilot signal for synchronization of the transmitter and receiver and then multiply the FM composite signal by a 38 kHz signal that is in phase with the original pilot. This multiplication process generates another composite signal having the L-R signal at the base band and having the L+R signal at the subchannel frequencies. The high frequencies of each composite signal are readily removed by filtering and the pilot is removed by a cancelling operation so that essentially the base band remains in each signal. To separate right from left the signals are added to yield a left channel signal and the signals are differenced to yield a right channel signal. If the composite signals are mismatched the left channel will have some right channel information in the output and the right channel will have some left channel information. Channel separation is a measure of how well the receiver has maintained each signal free of the other signal.
Heretofore it has been the practice to make separation adjustments by adjusting the gain of the L-R portion of the signal before it has been decoded. This requires a gain that is frequency dependent, resulting in circuit complexity and a loss of separation at high frequencies.